call static instead of self on protected method

[zearg]

Description

No linked issue.
When you want to extends PhpOffice\PhpWord\Shared\Html class, the protected method parseNode, will call "self::parseChildNodes" making the call parseNode useless in the new class. And prevent parseChildNodes from being customized in the new class.

The change made allows the parseNode method to look for parseChildNodes first in the newly created class

More concretely, you can now customize parseChildNodes by extending the Html class

Maybe we can do same thing with all static methods (replacing calls self:: by static::)

Checklist:

• I have run composer run-script check --timeout=0 and no errors were reported
• The new code is covered by unit tests (check build/coverage for coverage report)
• I have updated the documentation to describe the changes

[coveralls]

Coverage remained the same at 94.63% when pulling 2045e52 on zearg:develop into 1534dc2 on PHPOffice:develop.